kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 250 insertions, 0 deletions

diff --git a/usr/gen_initramfs.sh b/usr/gen_initramfs.sh
new file mode 100755
index 000000000..7eba2fddf
--- /dev/null
+++ b/usr/gen_initramfs.sh
@@ -0,0 +1,250 @@
+#!/bin/sh
+# Copyright (C) Martin Schlemmer <azarah@nosferatu.za.org>
+# Copyright (C) 2006 Sam Ravnborg <sam@ravnborg.org>
+#
+# Released under the terms of the GNU GPL
+#
+# Generate a cpio packed initramfs. It uses gen_init_cpio to generate
+# the cpio archive.
+# This script assumes that gen_init_cpio is located in usr/ directory
+
+# error out on errors
+set -e
+
+usage() {
+cat << EOF
+Usage:
+$0 [-o <file>] [-l <dep_list>] [-u <uid>] [-g <gid>] {-d | <cpio_source>} ...
+	-o <file>      Create initramfs file named <file> by using gen_init_cpio
+	-l <dep_list>  Create dependency list named <dep_list>
+	-u <uid>       User ID to map to user ID 0 (root).
+		       <uid> is only meaningful if <cpio_source> is a
+		       directory.  "squash" forces all files to uid 0.
+	-g <gid>       Group ID to map to group ID 0 (root).
+		       <gid> is only meaningful if <cpio_source> is a
+		       directory.  "squash" forces all files to gid 0.
+	-d <date>      Use date for all file mtime values
+	<cpio_source>  File list or directory for cpio archive.
+		       If <cpio_source> is a .cpio file it will be used
+		       as direct input to initramfs.
+
+All options except -o and -l may be repeated and are interpreted
+sequentially and immediately.  -u and -g states are preserved across
+<cpio_source> options so an explicit "-u 0 -g 0" is required
+to reset the root/group mapping.
+EOF
+}
+
+# awk style field access
+# $1 - field number; rest is argument string
+field() {
+	shift $1 ; echo $1
+}
+
+filetype() {
+	local argv1="$1"
+
+	# symlink test must come before file test
+	if [ -L "${argv1}" ]; then
+		echo "slink"
+	elif [ -f "${argv1}" ]; then
+		echo "file"
+	elif [ -d "${argv1}" ]; then
+		echo "dir"
+	elif [ -b "${argv1}" -o -c "${argv1}" ]; then
+		echo "nod"
+	elif [ -p "${argv1}" ]; then
+		echo "pipe"
+	elif [ -S "${argv1}" ]; then
+		echo "sock"
+	else
+		echo "invalid"
+	fi
+	return 0
+}
+
+print_mtime() {
+	local my_mtime="0"
+
+	if [ -e "$1" ]; then
+		my_mtime=$(find "$1" -printf "%T@\n" | sort -r | head -n 1)
+	fi
+
+	echo "# Last modified: ${my_mtime}" >> $cpio_list
+	echo "" >> $cpio_list
+}
+
+list_parse() {
+	if [ -z "$dep_list" -o -L "$1" ]; then
+		return
+	fi
+	echo "$1" | sed 's/:/\\:/g; s/$/ \\/' >> $dep_list
+}
+
+# for each file print a line in following format
+# <filetype> <name> <path to file> <octal mode> <uid> <gid>
+# for links, devices etc the format differs. See gen_init_cpio for details
+parse() {
+	local location="$1"
+	local name="/${location#${srcdir}}"
+	# change '//' into '/'
+	name=$(echo "$name" | sed -e 's://*:/:g')
+	local mode="$2"
+	local uid="$3"
+	local gid="$4"
+	local ftype=$(filetype "${location}")
+	# remap uid/gid to 0 if necessary
+	[ "$root_uid" = "squash" ] && uid=0 || [ "$uid" -eq "$root_uid" ] && uid=0
+	[ "$root_gid" = "squash" ] && gid=0 || [ "$gid" -eq "$root_gid" ] && gid=0
+	local str="${mode} ${uid} ${gid}"
+
+	[ "${ftype}" = "invalid" ] && return 0
+	[ "${location}" = "${srcdir}" ] && return 0
+
+	case "${ftype}" in
+		"file")
+			str="${ftype} ${name} ${location} ${str}"
+			;;
+		"nod")
+			local dev="`LC_ALL=C ls -l "${location}"`"
+			local maj=`field 5 ${dev}`
+			local min=`field 6 ${dev}`
+			maj=${maj%,}
+
+			[ -b "${location}" ] && dev="b" || dev="c"
+
+			str="${ftype} ${name} ${str} ${dev} ${maj} ${min}"
+			;;
+		"slink")
+			local target=`readlink "${location}"`
+			str="${ftype} ${name} ${target} ${str}"
+			;;
+		*)
+			str="${ftype} ${name} ${str}"
+			;;
+	esac
+
+	echo "${str}" >> $cpio_list
+
+	return 0
+}
+
+unknown_option() {
+	printf "ERROR: unknown option \"$arg\"\n" >&2
+	printf "If the filename validly begins with '-', " >&2
+	printf "then it must be prefixed\n" >&2
+	printf "by './' so that it won't be interpreted as an option." >&2
+	printf "\n" >&2
+	usage >&2
+	exit 1
+}
+
+header() {
+	printf "\n#####################\n# $1\n" >> $cpio_list
+}
+
+# process one directory (incl sub-directories)
+dir_filelist() {
+	header "$1"
+
+	srcdir=$(echo "$1" | sed -e 's://*:/:g')
+	dirlist=$(find "${srcdir}" -printf "%p %m %U %G\n" | LC_ALL=C sort)
+
+	# If $dirlist is only one line, then the directory is empty
+	if [  "$(echo "${dirlist}" | wc -l)" -gt 1 ]; then
+		print_mtime "$1"
+
+		echo "${dirlist}" | \
+		while read x; do
+			list_parse $x
+			parse $x
+		done
+	fi
+}
+
+input_file() {
+	source="$1"
+	if [ -f "$1" ]; then
+		# If a regular file is specified, assume it is in
+		# gen_init_cpio format
+		header "$1"
+		print_mtime "$1" >> $cpio_list
+		cat "$1"         >> $cpio_list
+		if [ -n "$dep_list" ]; then
+		        echo "$1 \\"  >> $dep_list
+			cat "$1" | while read type dir file perm ; do
+				if [ "$type" = "file" ]; then
+					echo "$file \\" >> $dep_list
+				fi
+			done
+		fi
+	elif [ -d "$1" ]; then
+		# If a directory is specified then add all files in it to fs
+		dir_filelist "$1"
+	else
+		echo "  ${prog}: Cannot open '$1'" >&2
+		exit 1
+	fi
+}
+
+prog=$0
+root_uid=0
+root_gid=0
+dep_list=
+timestamp=
+cpio_list=$(mktemp ${TMPDIR:-/tmp}/cpiolist.XXXXXX)
+# gen_init_cpio writes to stdout by default
+output=""
+
+trap "rm -f $cpio_list" EXIT
+
+while [ $# -gt 0 ]; do
+	arg="$1"
+	shift
+	case "$arg" in
+		"-l")	# files included in initramfs - used by kbuild
+			dep_list="$1"
+			echo "deps_initramfs := \\" > $dep_list
+			shift
+			;;
+		"-o")	# generate cpio image named $1
+			output="-o $1"
+			shift
+			;;
+		"-u")	# map $1 to uid=0 (root)
+			root_uid="$1"
+			[ "$root_uid" = "-1" ] && root_uid=$(id -u || echo 0)
+			shift
+			;;
+		"-g")	# map $1 to gid=0 (root)
+			root_gid="$1"
+			[ "$root_gid" = "-1" ] && root_gid=$(id -g || echo 0)
+			shift
+			;;
+		"-d")	# date for file mtimes
+			timestamp="$(date -d"$1" +%s || :)"
+			if test -n "$timestamp"; then
+				timestamp="-t $timestamp"
+			fi
+			shift
+			;;
+		"-h")
+			usage
+			exit 0
+			;;
+		*)
+			case "$arg" in
+				"-"*)
+					unknown_option
+					;;
+				*)	# input file/dir - process it
+					input_file "$arg"
+					;;
+			esac
+			;;
+	esac
+done
+
+# If output_file is set we will generate cpio archive
+# we are careful to delete tmp files
+usr/gen_init_cpio $output $timestamp $cpio_list